It is known to provide self-propelled trench-excavating machines having a chassis carrying a prime mover, such as an internal combustion engine, and mounted upon tracks which are driven indirectly by this prime mover, e.g. through a gear or hydraulic transmission. The chassis can carry a turntable which is rotatable about a vertical axis and on which a boom can be raised and lowered, i.e. swung about a horizontal axis.
At the end of the boom remote from the chassis, there may be swingably mounted a cutting head which can have, for example, a cutting tool rotatable about the axis of this head. The head may be swingable about a horizontal axis relative to the boom and hence the axis of the cutting tool may be varied in its orientation with respect to the ground in which the trench is to be excavated. For example, the cutting tool can be lowered into the ground to commence an excavation with its axis substantially vertical and then can be inclined to widen, lengthen or otherwise enlarge the trench. The soil and detritus loosened by the tool can be removed by a conveyor which can be positioned in the trench or hole.
A trench-digging machine of this type has been described in German utility model (Gebrauchsmuster) DT-GM 76 18 210. This apparatus is used to form the trench with its cutting tool held vertically. German utility model (Gebrauchsmuster) DT-GM 18 97 894 describes a machine in which the tool can assume a given angle relative to the tracks of the vehicle.
With such systems, a difficulty has been uncovered when it is desired to propel the vehicle with the tool in engagement with the ground. If the tool is vertical, for example, any movement of the chassis by driving the tracks puts a considerable strain on the boom and cutting head and reduces the excavating efficiency since normally only the lower end of the tool can effect a maximum degree of cutting, the remainder of the tool forming primarily a detritus-removal operation or, at best, operates as a cutter with reduced effectiveness.